1. Field of the Invention
The present invention relates to x-ray tubes, and in particular to an x-ray tube having a cathode head containing an electron source mounted on a supporting element opposite and anode.
2. Description of the Prior Art
An x-ray tube having a cathode head with a thermionic coil or filament contained in a recess of the cathode head for directing an electron beam at an anode is disclosed in U.S. Pat. No. 3,433,955. This x-ray tube is typical of conventional x-ray tubes wherein the electron beam issuing from the cathode in the form of a filament is directed by means of a focusing device such as a focusing cup or Wehnelt cylinder, to the focal spot on the anode. The thermionic cathode generally consists of a wire wound in the form of a helical spring or coil. In order to focus the electron beam, the thermionic cathode generally is disposed with a recess of the metallic cathode head, the recess being at the same potential as the cathode head. The electron beam is thus held together and shaped in a desired manner.
In particular, for rotary anode x-ray tubes, a rectangular focal spot is generated which originates from the thermionic coil which is disposed in a groove-like recess of the cathode head. As described in the aforementioned U.S. Pat. No. 3,433,955, the anode structure consists of nickel or similar material.
A problem in the design of such x-ray tubes is to achieve improved focusing, in particular in the case of small focal spots, and to increase the focus stability during operation.